A bioenergetic model for sea otters : exploring interactions between energy requirements and dietary choices

Sea otters (Enhydra lutris) are keystone predators with voracious appetites that impose immense predation pressure on their prey, thereby significantly altering the structure of local ecosystems. To better understand how much food sea otters must consume to meet their energy needs and impacts of predation, I developed bioenergetic models for sea otters. These models infer how the energy expenditures of individual sea otters vary under changing conditions, while accounting for sex, age, and reproductive status. I created three models— females without a pup, females with a pup, and males— using peer-reviewed, published data on sea otter ecology, physiology, and behaviour. The models demonstrated that females caring for a pup have the highest total energy expenditures when compared to adult males (3-10% less on average) and females without pups (33% less). Sensitivity analyses showed that the body mass parameter had the largest effects on model outcomes, due to many input values being mass-specific. The second variable affecting model outcomes was metabolic rate, which helps inform which types of data should be collected to improve model accuracy. The amount of food sea otters require to satisfy their energy expenditures depends on complex relationships between the energy density, digestive value, and foraging costs of obtaining different prey items. I converted the energy expenditure estimates generated by the models into ingested food mass (IFM) and biomass removed to investigate the effects of diet and prey availability on prey consumed by sea otters. I chose five diet scenarios to compare: one general otter diet and four specialist diets. Otters that prefer more energy dense prey species (e.g., abalone) had lower IFM than otters consuming less dense prey (e.g., urchins). The amount of food required by mothers with pups on a low energy diet may present a significant challenge. I further explored how changes in food availability affect otters’ foraging efficiency and found that as foraging efficiency decreases, foraging time and energy expenditure both increase. Therefore, prey selection by sea otters has critical energetic consequences effecting their overall energy balance, and may contribute to limiting range expansion or population growth of sea otters in certain areas.